1. Field of the Invention
This invention relates to a powder coating composition and more particularly, to a powder coating composition comprising an epoxy resin.
2. Discussion of the Prior Art
Many electrical devices require some type of external protective means to help protect fragile and sensitive electrical components from harmful mechanical shock and from detrimental effects of an external atmosphere, such as the effects of moisture and other atmospheric contaminants, which may adversely affect the electrical characteristics of such devices. A protective means such as an exterior continuous protective coating layer composed of a suitable resin, either a thermosetting or thermoplastic resin, helps protect such electrical devices from the named effects. A continuous resin protective coating layer is advantageous because, typically, such a layer is electrically insulative and has good resistance to mechanical shock due to its impact strength, and it is able to provide, under many operating conditions, suitable protection from contaminants that may be present in the atmosphere.
While such a continuous protective coating layer helps to protect the electrical components of the device from shock and the harmful effects of the surrounding external atmosphere, the protective coating layer may introduce new harmful effects to the satisfactory performance of the device. Such effects may include shrinkage of the protective coating layer upon curing or hardening resulting in harmful pressures being exerted on the electrical component. Also, if the coefficient of thermal expansion for the electrical component and the protective coating layer are not substantially equal, such as the coefficient for the protective coating layer being less than that of the components, additional harmful compressive pressures can be exerted against the electrical component during subjection to elevated temperature cycling. Thus, the electrical component would tend to expand at a faster rate than the protective coating layer when the temperature of the device was increased, thereby causing the layer to exert pressure against the component. If the coefficient of thermal expansion of the protective coating layer is greater than that of the component, the protective coating layer may exert harmful compressive pressures against the component when the temperature of the device was decreased below ambient temperatures.
The adhesive nature of the protective coating layer to the surface of the electrical component, while desirable for good protection of the electrical component by maintaining the continuous nature of the coating, can have a detrimental effect on the electrical component due to adhesion of the protective coating layer to the surface of the component during a temperature cycling which may create harmful stresses at the interface between the layer and the surface of the device. These stresses are caused by an expansion of the protective coating layer at a rate different from the expansion rate of the electrical component and the adhesion of the protective coating layer creating tensional forces at the surface of the component during temperature cycling.
Any of the above stresses may result in harmful spalling, cracking or fracturing of the protective coating layer or of the electrical component or both to relieve built up pressures. Such effects may reduce the ability of the device to withstand further temperature cycling, may harmfully affect the electrical characteristics of the device, break the seal allowing harmful atmospheric contamination or, perhaps, even be a cause of a complete failure of the device, as well as causing failure of the insulative properties of the protective layer.
A protective coating which can be readily applied, as by powder coating techniques, which eliminates or at least minimizes the above-described stresses is needed and is desired.